Electron Paramagnetic Resonance (EPR) spectroscopy and imaging techniques are being increasingly applied to study biological (aqueous) systems. Exciting possibilities for developing new, non-invasive diagnostic tools have stimulated interest in EPR techniques.
One property of biological systems is that electromagnetic radiation above about 500 MHz is strongly absorbed due to dielectric effects in the aqueous medium. Accordingly, there has been a spurt in attempts to develop instrumentation for the study of paramagnetic systems at radiofrequencies (&lt;500 MHz). Large volume loop-gap, bird-cage and other classes of resonators which are routinely used in NMR spectroscopy and imaging can be directly employed for both constant wave (CW) and pulsed RF EPR spectroscopy and imaging. The relatively low Q and high filling factors are ideally suited for time domain experiments.
Existing RF EPR imaging utilize CW techniques and thus the excitation signal is inherently a narrow band width signal. However, EPR imaging requires a non-homogenous internal magnetic field which spreads the resonance frequency over about an 100 MHz band width. As is well-known from Fourier analysis, a carrier frequency of 300 MHz requires a 10 ns pulse envelope to cover 100 MHz. However, the shortest reasonable pulse duration RF spectroscopy is approximately 25 ns, resulting in only about a 40 MHz frequency spread, due to rise time limitations of the pulse and diode switching. Short duration pulses are also required to perform very fast signal averaging in the time domain.